A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
A lithographic apparatus usually uses a radiation beam to project the pattern from the patterning device onto the substrate. Properties of the radiation beam may be controlled by the lithographic apparatus. The properties may be adjusted, for example depending upon the type of pattern to be projected onto the substrate. A lithographic apparatus may modify the polarization of the radiation beam. A lithographic apparatus may comprise various detectors to measure properties of the radiation beam. For example, a lithographic apparatus may have a detector used to measure the intensity of the radiation beam.